Existing SLM devices use a number of different designs including multiple-quantum-well structures, liquid crystals, deformable mirrors, pockels-type electro-optic arrays such as the known PROM device, e-beam DKDP light valves and Kerr-type electro-optic arrays such as transverse PLZT arrays. However, it will be appreciated that some applications of these devices have simultaneous requirements on wavelength coverage, fill-factor, refresh-rate, and damage optical fluence, which are not met by any of the existing devices. Also, most existing SLMs are quite expensive, which prevents wide-spread application.
PLZT material is a ceramic which is very attractive for large arrays because the material is rugged, easy to fabricate in large pieces, and because several machining/fabrication techniques are available.
A transverse electro-optic effect in perovskite-like ceramics such as PLZT and PZT is well known. U.S. Pat. No. 4,636,786, issued Jan. 13, 1987, entitled "Electrooptic Ceramic Display and Method for Making Same" to Haertling describes a device using a transverse electric field in a ferroelectric PLZT ceramic.
With the known transverse electro-optic effect, the electric field and light propagation direction are orthogonal. In contrast, a longitudinal effect means that the electric field lines due to the applied voltage are parallel to the propagation direction of the light beam.
The transverse field in these materials creates a birefringence which can be used to produce an amplitude or phase modulator. The birefringence can either be linear (Pockels-effect) or quadratic (Kerr effect) with the applied electric field.
U.S. Pat. No. 3,741,627, issued Jun. 26, 1973, entitled "Strain Biased Ferroelectric Electro-Optics" Haertling et al. contemplates a longitudinal effect in a ferroelectric PLZT ceramic, but requires that a strain be introduced by the mechanical mounting of the ceramic in a stressed condition.
U.S. Pat. No. 3,744,875, issued Jul. 10, 1973, entitled "Ferroelectric Electrooptic Devices" Haertling et al. describes a longitudinal device which make use of the scatter of light from the ceramic under a longitudinal field to amplitude modulate the amount of light transmitted by the ceramic.
In Japanese Patent No. 58-130321 (A) issued Aug. 3, 1983, entitled "Optical Switching Array Element", there is described an array in which grooves are used to prevent light scattered from one element being transmitted to an adjacent element to prevent optical cross-talk between the elements of the array.
The major drawbacks of the transverse design are the limited interaction length due to the limited penetration of the fringing electric fields (typically 50 micrometers), and the reduced filling factor.
For applications such as holographic phase conjugation, phase modulators are superior to amplitude modulators because of their much higher diffraction efficiency.